1. Field of the Invention
The present invention relates to an optical disk apparatus that records and/or reproduces information to/from an optical recording medium by using near-field light. The present invention also relates to a servo control method used for the optical disk apparatus.
2. Description of the Related Art
In the related art, near-field light obtained by a solid immersion lens such as an SIL (Solid Immersion Lens) formed by a two-group lens or an SIM (Solid Immersion Mirror) formed as a single lens is often utilized as recording and/or reproduction means. The use of such near-field light makes it possible to achieve a finer spot diameter to meet the need for higher density optical disks. The SIL, which is interposed between a condenser lens and an optical disk as an optical recording medium, is a lens of a high refractive index shaped like a spherical lens that is partially cut away. The SIL is arranged such that its spherical surface faces the condenser lens side and its surface opposite to the spherical surface faces the signal recording surface of the optical disk. By interposing such an SIL between the condenser lens and the optical disk, a numerical aperture larger than the numerical aperture of the condenser lens can be realized, which makes it possible to achieve a finer spot diameter as described above.
To perform recording/reproduction to/from an optical disk by using an SIL, it is necessary to condense recording laser onto an end face of the SIL, bring the end face of the SIL (surface of the SIL opposed to the optical disk) and the optical disk close to each other to a distance at which near-field light is generated (¼ of the wavelength of light or less, typically about 100 nm or less), and keep that distance constant to thereby keep the size of the condensed light spot on the optical disk constant.
Japanese Unexamined Patent Application Publication No. 2001-076358 discloses a technique for keeping the distance between an SIL end face and an optical disk at a predetermined distance. As the gap control for controlling the distance between the SIL end face and the optical disk to be constant, the technique includes first optical means for condensing light to be radiated onto an optical recording medium, and second optical means interposed between the first optical means and the optical disk to realize a numerical aperture larger than the numerical aperture of the first optical means, and the distance between the second optical means and the optical disk is kept at a predetermined distance by using the linear characteristic of the quantity of total-reflection return light from the optical disk within the near-field region between an end face of the second optical means and the optical disk (specifically, the quantity of return light from an irradiation surface opposed to the optical disk).
One type of such a near-field optical disk apparatus is an apparatus that uses only one wavelength. That is, the near-field optical disk apparatus of this type is one in which a gap-servo laser light source and a read/write laser light source are the same single laser light source.